There's a PortableApps version of Raw Therapee - unfortunately they used a pretty unstable version of RT for the build.

It's also pretty easy to make a standalone version of RT simply by downloading the latest version and then using UniExtract to pull all the necessary files out of of the MSI installer (this is what I've got on my USB stick).

$25 will get you a copy of Nama5, which is a standalone converter - it's not bad, but its highlight recovery is very poor, and it applies a preset amount of sharpening to files, and you can't (as far as I've been able to figure) turn it off. Even so, it might do the job.